Subdivison surface awkward when extruding
When you extruded, as you didn't activate the Clipping option of the Mirror modifier, you created a inner face on the mirror axis.

So you can either:


Cancel your extrusion and redo it with the Clipping option enabled
Or delete the inner face: On the bottom of your 3D view, click Face Select button, disable the Limit Selection to Visible button, you'll see the inside face, then select it and X > Delete Faces.


Also, you should put the Mirror modifier above the Subsurface modifier.